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Layer 3 Switching

The term Layer 3 switching makes many people’s eyes glazeover. In this module, we’ll explain what Layer 3 switching is and how itcompares with Layer 2 switching and routing.

Contents

-WhatIs Layer 3 Switching?

-PacketManipulation at Layer 3

- DifferenceBetween Layer 2 Switching, Layer 3 Switching, and Routing


What Is Layer 3 Switching?

Recently, the industry has been bombarded with terminologysuch as Layer 3 switching, Layer 4 switching, multilayer switching, routing switches,switching routers, and gigabit routers. This “techno-jargon” can beconfusing to customers and resellers alike.

For purposes of this discussion, all these terms essentiallyrepresent the same function, and, as such, the term Layer 3 switching is usedto represent them all. While the performance aspect of Layer 3 switching makesmost of the headlines, higher performance in switching packets does not, by itself,promise that all problems are solved in a network. There must be a recognitionthat application design, mix of network protocols, placement of servers, placementof networking devices, management, as well as the implementation of end-to-endintelligent network services are at least as important—maybe more so—thansimply adding more bandwidth and switching capability to the network.

Why Do We Need Layer 3 Switching?

So, why do we need Layer 3 switching? Enterprise networks faceunprecedented challenges today. Desktop computing power has tripled in the pasttwo years and shows no sign of leveling off. The proliferation of network-dependentintranet and multimedia applications has increased traffic volumes in many campusnetworks by an order of magnitude over the past several years. Network managershave responded to this need to move data at greater speeds by moving more desktopsto switched 10/100 Mbps and deploying LAN switching at unprecedented levels, bothin the data center and in the wiring closets to scale their end-to-end bandwidth.To effectively utilize the increased capacity, they must scale their Layer 3 performanceto handle changing traffic patterns. Conventional wisdom that 80 percent of thetraffic stays local to the subnet and 20 percent or less traverses across subnetsno longer holds. More than half of the traffic volume travels across subnet boundaries.Two factors contribute to these changing traffic patterns.
With Web-based computing, a PC can be both a subscriber and a publisher of information.As a result, information can now come from anywhere in the network, creating massiveamounts of traffic that must travel across subnet boundaries. Users hop transparentlybetween servers across the entire enterprise by using hyperlinks, without theneed to know where the data is located.
The second factor leading to the loss of locality is the move toward server consolidation.Enterprises are deploying centralized server farms because of the reduced costof ownership and ease of management. All traffic from the client subnets to theseservers must travel across the campus backbone, exacerbating performance problems.
Because of the rising levels of anywhere-to-everywhere communication, Layer 3switching that can scale with increasing link speeds has become an imperative.Layer 3 switching is required to meet the demands of both client/server and peer-to-peertraffic on the intranet.

What Is Layer 2 Switching?

What is the difference between a Layer 2 switch, a Layer 3switch, and a router?

A Layer 2 switch is essentially a multiport bridge. Switching and filtering arebased on the Layer 2 MAC addresses, and, as such, a Layer 2 switch is completelytransparent to network protocols and users’ applications.
Layer 2 switching is the number one choice for providing plug-and-play performance.


What Is Routing?

In contrast to Layer 3 switches, routers make Layer 3 routingdecisions by implementing complex routing algorithms and data structures in software.Keep in mind this has little to do with the forwarding aspects of routing.
Routing has two basic functions, path determination, using a variety of metrics,and forwarding packets from one network to another.
The path determination function enables a router to evaluate the available pathsto a destination and to establish the preferred handling of a packet.
Data can take different paths to get from a source to a destination. At Layer3, routers really help determine which path. The network administrator configuresthe router enabling it to make an intelligent decision as to where the routershould send information through the cloud.
The network layer sends packets from source network to destination network.
After the router determines which path to use, it can proceed with switching thepacket: taking the packet it accepted on one interface and forwarding it to anotherinterface or port that reflects the best path to the packet’s destination.

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